In communications and radar systems applications, it is often desirable to control radio frequency (RF) signals with a variety of RF devices. Tunability of RF devices at microwave and millimeter wave frequencies is desirable for a variety of civilian and military applications. It has been recognized that the integration of ferrimagnetic, ferromagnetic and superconductor materials in microstrip configurations could improve tunable devices by providing the device with new capabilities such as lower loss and simpler geometries that reduce size and cost.
A ferromagnetic material (also referred to as a “ferromagnet”) is a substance (e.g. iron, nickel cobalt, other metals and various alloys) that exhibits extremely high magnetic permeability, the ability to acquire high magnetization in relatively weak magnetic fields, a characteristic saturation point, and a magnetic hysteresis. A ferrimagnetic material (also referred to as a “ferrite”) is a substance (e.g. iron oxides) that possesses magnetic properties comparable in some respects to the magnetic properties of ferromagnetic substances. Although the magnetic strength of ferrites tends to be weaker than that of the ferromagnetic metals, an important and distinguishing feature of ferrites is that they exhibit a dielectric or electrical insulating property. For this reason, ferrites are particularly well suited for applications where electrical conduction is to be avoided.
Ferrimagnetic and ferromagnetic material (also referred to as spontaneous magnetic material) are also gyrotropic media that can influence the propagation of an electromagnetic wave or signal. If the electromagnetic wave has a relatively high frequency, including a frequency in the microwave and millimeter wave frequency bands, a gyromagnetic interaction occurs between the magnetization of the spontaneous magnetic material and the magnetic field component of the electromagnetic wave of the proper polarization traversing the spontaneous magnetic material. At a specific frequency that is proportional to the strength of the internal magnetic field, the interaction becomes resonant and the electromagnetic wave undergoes dispersion and absorption by the spontaneous magnetic material across a narrow band about the resonance frequency. At frequencies away from the gyromagnetic resonance condition, the absorption becomes negligible but a dispersion effect remains in the wave. This dispersion causes a change in the velocity of propagation that produces phase shift in the electromagnetic signal. This property is utilized in phase shifters, switchable circulators and tunable filters. The absorption near resonance is utilized in other devices such as switches, variable attenuators, and tunable absorption filters.
The amount of gyromagnetic interaction is proportional to the magnetization in the spontaneous magnetic material whether at resonance or away from resonance. Magnetization in a conventional polycrystalline ferrite structure exhibits hysteresis. The term hysteresis means that changes in the magnetic state of the spontaneous magnetic material structure induced by a magnetic field are not directly reversible by removal of the field. For this reason, the shape and stability of the hysteresis loop are of critical importance to device performance that depends on a variable magnetization at low magnetic fields.
Polycrystalline materials are dense and comprise many individual crystals usually, but not necessarily, of random crystallographic orientation. Modem polycrystalline microwave magnetic devices are commonly operated in a remanent state and are designed to accommodate the hysteresis loop phenomenon. An initial negative magnetic field pulse drives the device into reverse magnetic saturation and a second positive magnetic field pulse selects an appropriate magnetization level of a minor hysteresis loop such that when the second pulse is removed, the device settles into a desired remanent magnetization.
This technique to obtain a desired remanent magnetization suffers from several limitations. First, it requires a look-up table to determine appropriate magnetic field pulse strength to cause the device to settle into a particular magnetization. Second, devices provided from polycrystalline materials suffer from high coercivity and therefore, energy is wasted when switching between magnetization states. Third, the hysteresis characteristics of such devices require relatively large amounts of energy to reset the device into saturation. Fourth, the switching time between pulses cannot be reduced below several microseconds without utilizing current drive pulses having relatively high current levels. Magnetic saturation is necessary in order to achieve a full range of tunability. Magnetic saturation further requires a relatively large amount of current and inductance in the magnetizing driver circuit.
One method for greatly reducing the inefficiencies and uncertainties introduced by the hysteresis loops exhibited by polycrystalline devices is the use of single-crystal ferrite structures. A single-crystal material has distinct preferred directions of magnetization uniformly throughout the material and exhibits virtually no hysteresis in its magnetization curve. In single-crystal devices the magnetization can be crystallographically aligned with the preferred directions, in other words along the “easy” axes, in order to eliminate, or nearly eliminate, the hysteresis loop. This leads to a device which exhibits negligible coercivity and therefore has a magnetization which is nearly directly reversible. For single-crystal devices, departure from alignment with the easy axis increases the energy required to magnetize the material.
To overcome some of the limitations described above, frequency tuning in recent microwave ferrite resonators and filters having planar geometries is accomplished by varying the magnetization vector magnitude and direction relative to the RF signal propagation using relatively complicated magnetic structures. The magnetically tunable resonator shown in FIG. 1 is an example of one such structure.
The resonator shown in FIG. 1 is described in U.S. Pat. No. 6,141,571, issued to Dionne on Oct. 31, 2000 and assigned to the assignee of the present invention and hereby incorporated herein by reference in its entirety. Briefly, however, as shown in FIG. 1, magnetic tunability requires a single-crystal or quasi-single crystal ferrite 75 with additional structures including a demagnetizing gap 46, a wire coil 45, circuitry and power to generate a magnetic field H and a magnetization M. The requirement of additional external magnetic circuits increases the device cost and size, and the limitations on the magnetic structure cause fabrication and packaging problems in certain applications in which a relatively high level of integration is required. The magnetic structure is limited in some applications to either a continuous closed-loop configuration, for example in the shape of a toroid or a “window-frame” configuration. The external magnetic field H can interfere with the circuit performance of circuits having RF conductors fabricated from superconductor materials in certain applications. In addition, the speed at which the magnetization M can be switched in the ferrite device is somewhat limited by hysteresis and inductance. The concept of magnetically tuning ferrite resonators by applying a magnetic field to magnetize the ferrite is described in detail in the aforementioned U.S. Pat. No. 6,141,571.
It would, therefore, be desirable to provide a method and apparatus to control the gyromagnetic interaction between an RF signal and a magnetic structure without having to magnetize the magnetic structure. It would be further desirable to provide a tunable resonator which does not require additional external magnetic circuits or have limitations on the magnetic structure configuration.